1. Field of the Invention
The present invention relates generally to an apparatus for particle synthesis and more particularly to an apparatus capable of clean high temperature synthesis of small particles and nanoparticles.
2. Technical Background
Over the years, there has been rapid progress in the areas of electronics, materials science, and nanoscale technologies resulting in, for example, smaller devices in electronics, advances in fiber manufacturing and new applications in the biotechnology field. The ability to generate increasingly smaller, cleaner and more uniform particles is necessary in order to foster technological advances in areas which utilize small particulate matter. The development of new, efficient and adaptable ways of producing small particulate matter becomes more and more advantageous.
The size of a particle often affects the physical and chemical properties of the particle or compound comprising the particle. For example, optical, mechanical, biochemical and catalytic properties often change when a particle has cross-sectional dimensions smaller than 200 nanometers (nm). When particle sizes are reduced to smaller than 200 nm, these smaller particles of an element or a compound often display properties that are quite different from those of larger particles of the same element or compound. For example, a material that is catalytically inactive in the macroscale can behave as a very efficient catalyst when in the form of nanoparticles.
The aforementioned particle properties are important in many technology areas. For example, in optical fiber manufacturing, the generation of substantially pure silica and germanium soot particles from impure precursors in a particular size range (about 5-300 nm) has been key in providing optical preforms capable of producing high purity optical fiber. Also, in the field of pharmaceuticals, the generation of particles having certain predetermined properties is advantageous in order to optimize, for example, in vivo delivery, bioavailability, stability of the pharmaceutical and physiological compatibility. The optical, mechanical, biochemical and catalytic properties of particles are closely related to the size of the particles and the size of the compounds comprising the particles. Gas-phase methods of particle generation are attractive, since gas-phase methods typically yield large quantities of high purity particles which are within a desirable size range.
Particle generators such as aerosol reactors have been developed for gas-phase nanoparticle synthesis. Examples of these aerosol reactors include flame reactors, tubular furnace reactors, plasma reactors, and reactors using gas-condensation methods, laser ablation methods, and spray pyrolysis methods. In particular, hot wall tubular furnace reactors have proven adept for soot particle generation for silica preform production in optical fiber manufacturing. Hot wall tubular furnace reactors normally use resistive heating elements or use burners to supply energy to reactor walls near the reaction zone.
Induction Soot Generators (ISGs) are examples of hot wall tubular furnace reactors using inductive heating elements to heat the reactor walls. Examples of such ISGs developed for synthesis of silica soot particles for use in optical fiber manufacturing are described in commonly owned US Patent Application Publication 2004/0206127 the disclosure of which is incorporated herein by reference in its entirety. The ISGs described in that reference have inductively heated reactor walls typically made of platinum, rhodium, or a platinum\rhodium compound. A description of one embodiment of an ISG in that reference also shows the use of Radio Frequency (RF) electromagnetic energy to heat certain portions of the reaction zone, and mentions the possible use of graphite as a suitable RF susceptor. ISGs have a number of advantages over other tubular soot generators. For example, combustion is not needed for supplying the energy to heat the reactor walls of the reaction zone in order to support the chemical reaction. Also, there is an increased ability to control the process temperature including the reaction temperature due to the increased control of the energy source as compared to generators using burner heating of the walls of the reaction zone.
However, ISGs do have some disadvantages. For example, the reactor walls of the reaction zone may become damaged due to exposure of the reactor walls to aggressive chemicals, such as chlorine (Cl) and oxygen (O) ions at high temperatures (above 1500° C.). These aggressive environmental conditions are damaging even for reactor walls made from platinum, rhodium, or a platinum\rhodium compound. As a result, the mechanical and induction properties of the reactor walls deteriorate over time. Also, this degradation of the reactor wall materials allows platinum and rhodium compounds to contaminate the synthesized particles. When degradation occurs, the reactor wall material must be replaced, which is both costly and time consuming. It would be advantageous to develop an apparatus capable of high temperature particle synthesis where degradation of the reactor walls is minimized and if any degradation occurs, contamination would be isolated from the reaction area.